A basic principle behind an Orthogonal Frequency Division Multiplexing (OFDM) entails dividing high rate data stream into a large number of slow rate data stream and transmitting simultaneously these slow rate data stream using a plurality of carriers. Here, each of the plurality of carriers is referred to as a subcarrier. In the OFDM system, the plurality of carriers enjoys orthogonality with each other. As such, even if the frequency elements of the carriers overlap, it is possible for the receiving end to detect the signals. In addition, the data stream having high data rate are passed through a serial-to-parallel converter and are converted into a plurality of low rate data stream. The converted plurality of data stream is multiplied by each subcarrier and thereafter, each data rate is combined with each other before being transmitted to the receiving end.
The plurality of parallel data stream generated by the serial-to-parallel converter can be transmitted on the plurality of subcarriers after being processed by Inverse Discrete Fourier Transform (IDFT). The IDFT can be applied an Inverse Fast Fourier Transform (IFFT) for efficient reconstruction.
Since a symbol duration of the subcarriers having low data rate increases, relative signal dispersion caused by multi-path delay spread in relation to time decreases. It is possible to reduce Inter-Symbol Interference (ICI) by interjecting a guard interval whose length is longer than that of the length of channel delay spread between OFDM symbols. In addition, a part of the OFDM signal can be duplicated and placed at the starting portion of the symbol in the guard interval. As a result, the OFDM symbol can become cyclically extended and protect the symbol.
Hereafter, a conventional Discrete Fourier Transform Spreading OFDM (DFT-S-OFDM) scheme will be explained. The DFT-S-OFDM scheme can also be referred to as a Single Carrier-Frequency Division Multiple Access (SC-FDMA). The conventional SC-FDMA scheme is usually applied in the uplink. In operation, spreading is applied using the DFT matrix in the frequency domain prior to generating the OFDM signal. Thereafter, the output is modulated according to the conventional OFDM scheme and then transmitted.
In the SC-FDMA scheme, the data symbols are spread by the DFT matrix before being transmitted. In the following equation, ‘N’ represents a number of the subcarriers used to transmit the OFDM signals, ‘Nb’ represents a number of subcarriers for a temporary user, ‘F’ represents the DFT matrix, ‘s’ represents a data symbol vector, ‘x’ represents a data spread vector in the frequency domain, and ‘y’ represents an OFDM symbol vector transmitted in the time domain. Based on these elements, the SC-FDMA scheme can be explained according to Equation 1.x=FNb×Nbs  [Equation 1]
In Equation 1, FNb×Nb is a DFT matrix, whose size is represented by Nb, used for spreading the data symbols. Subcarrier mapping is performed according to a subcarrier allocation method on vector x which is the vector spread by the DFT matrix. Thereafter, the data spread vector is converted from the frequency domain to time domain to acquire the signal for transmitting to the receiving end. The equation related to the transmission signal for the receiving end is as follows.y=FN×N−1x  [Equation 2]
In Equation 2, FN×N represents a DFT matrix having a size of N used for converting the signal of the frequency domain to time domain signal. The signal y generated in the course of the procedure is applied a cyclic prefix thereto and then transmitted. Here, the scheme used for generating and transmitting the signal to the receiving end is based on the SC-FDMA scheme. The size of the DFT matrix can vary depending on use and/or purpose. For example, if the size of the DFT matrix is same as the number of IDFT points, a Peak-to-Average Power Ratio (PAPR) of the transmitting end can be reduced.
Hereafter, the OFDM Access (OFDMA) will be explained. The OFDMA is a multiple access scheme which provides a part of available subcarriers to each user using a modulation system employing an orthogonal plurality of subcarriers. The OFDMA provides frequency resources (e.g., subcarriers) to each user. These frequency resources are independently provided to a plurality of users and thus conflict between users can be generally avoided.
Hereafter, general OFDMA transmitting/receiving devices are explained. FIG. 1 is a block diagram illustrating downlink transmitting/receiving ends according to a conventional art.
In the transmitting end, a bit stream is mapped according to a constellation mapping scheme using modulation techniques such as Quadrature Phase Shift Keying and 16 Quadrature Amplitude Modulation. That is, the bit stream is mapped as a specific data symbol, and the data symbol is converted into a parallel data symbol after passing through a serial-to-parallel converter. In the conversion, a number of converted data symbols (i.e., Nu(n)) that are converted correspond directly to a number of subcarriers allocated to each user (n). As illustrate in FIG. 1, the bit stream for user 1 is converted to parallel data symbols Nu(1) number of allocated subcarriers. The number of subcarriers allocated to each user can be same or different. Furthermore, the data symbol size (Nu(n)) for each user can be same or different as well.
The converted parallel data symbols are mapped to Nu(n) number of subcarriers which is allocated to nth user out of Nc number of subcarriers. The remaining (Nc-Nu(n)) number of subcarriers are mapped to other users. Using a symbol-to-subcarrier mapping module, un-allocated subcarriers are padded with ‘0’ (e.g., zero padding). The resulting output of the symbol-to-subcarrier mapping module is thereafter inputted into a Nc-Point Inverse Fast Fourier Transform (IFFT) module.
In attempt to reduce Inter-Symbol Interference (ISI), the output of the IFFT module is first added to a cyclic prefix and passed through a parallel-to-serial converter module before being transmitted.
The conventional OFDMA receiving device operates in reverse order from that of the transmitting device. More specifically, the received data symbols are passed through the serial-to-parallel converter module, followed by the Nc-Point FFT, before being processed at the subcarrier-to-symbol mapping module. Thereafter, the data symbols are decoded by the constellation demapping module.
Hereafter, resource allocation in the OFDMA system will be explained. In the entire frequency broadband, specific frequency resources (e.g., subcarriers) are allocated to specific users. In other words, the allocated frequency resources are not shared with another user. More specifically, in allocating the frequency resources to the user(s) in the OFDMA, information on subcarrier allocation has to be sent to a mobile station (MS) from a base station (BS) so that the users can receive the allocated frequency resources. If allocation information for all of the subcarriers to be sent to the MS, the amount of information would be too large, and to minimize the allocation information being sent, a plurality of subcarriers can be grouped into ‘chunks’ before being sent to the MS.
FIG. 2 illustrates allocating subcarriers in units of chunks to a specified user. As illustrated in FIG. 2, there are various allocation methods by which to allocate the subcarriers in chunks, including a distributed allocation method and a localized allocation method, for example. Each arrow represents chunks, which is comprised of a plurality of grouped subcarriers.
The distributed allocation method of FIG. 2 allocates the chunks provided across the broadband of the authorized communication system to a specified user when a specified number of chunks from total number of chunks are allocated. By allocating the chunks provided across the broadband to the specified user, diversity gain can be achieved in the frequency domain. The allocation method of FIG. 2 depicts grouping chunks that are close to each other (neighbor chunks) and allocating the chunks to the specified user.
FIG. 3 illustrates a distributed allocation method and a localized allocation method. More specifically, FIG. 3 illustrates the distributed allocation method and the localized allocation method in an environment where there are a total of 32 chunks while 10 chunks are allocated to UE1, 8 chunks are allocated to UE2, and 14 chunks are allocated to UE3. Here, the chunk indices are different according to each allocation method. The distributed allocation method (a) of FIG. 3 represents an index with randomly allocated chunks. In addition, the localized allocation method (b) of FIG. 3 represents an index with chunks grouped in groups.
If the BS and the MS are sharing the indices, the allocation information that needs to be provided by the BS to the MS for proper reception is the available index per user and whether a distributed or localized allocation is used. In detail, for UE1, which receives data via 0-9 chunks, a first/start index 0 and a last/end index 9 are transmitted. Moreover, for UE2, which receives data via 10-17 chunks, a first index 10 and an last index 17 are transmitted. Furthermore, for UE3, which receives data via 18-31 chunks, a first index 18 and a last, index 31 are transmitted. In the transmission of the first and last indices, information on whether localized chunks or distributed chunks are used is transmitted to each UE.